Vertical transmission of St. Louis encephalitis virus was demonstrated by a strain of Aedes atropalpus mosquitoes from Garrett County, MD. Five-day-old parental females containing fully formed autogenous eggs were infected by intrathoracic inoculation. St. Louis encephalitis virus was detected in batches of fertilized eggs and fourth instar larvae. Estimated filial infection rates of approximately 1% were observed for both eggs and larvae.